It is a constant endeavor to find improved techniques of placing a visible or invisible identifying mark on an image. This is generally useful to establish ownership, origin and authenticity, and also to discourage those who might wish to purloin or misappropriate the work. Identifying marks are also useful to give evidence of unauthorized alteration or disclosure. Visible marks are herein classified as being either visible robust or visible fragile. A mark is classified as visible robust if it can be seen by the unaided eye and cannot be easily removed from the work-piece, if at all, without leaving telltale evidence. It is classified as visible fragile if the mark itself is visibly altered by an attempt to alter the work-piece or its wrapper.
Invisible marks are herein classified relative to the appearance of that mark to a human being with normal visual acuity. A mark on an image is classified as having an invisibility classification level of undetectably invisible if, when the image without the marking is displayed together with an image copy with the marking, the human being is equally likely to select either of these copies. An undetectably invisible mark is below or at the human being's just noticeable difference. A mark on an image is classified as having an invisibility classification level of subliminally invisible if the mark is not distracting to the human viewer, although it is above the human being's just noticeable difference. An image mark is classified as being marginally invisible if it does not cause the marked image to lose its usefulness or value because of the mark. An image marking is classified as being poorly invisible if the marking causes a reduction in the usefulness and/or value of the image.
Presently, both visible and invisible markings of hardcopy documents are used as a generally dependable method of establishing ownership and authenticity. These time-tested methods are also useful for marking a “softcopy” digitized image, also referred to herein as an image. A digitized image is an abstraction of a physical image that has been scanned and stored in a computer's memory as rectangular arrays of numbers corresponding to that image's (one or more) color planes. Each array element corresponding to a very small area of the physical image is called a picture element, or pixel. The numeric value associated with each pixel for a monochrome image represents the magnitude of its average brightness on its single color (black and white) plane. For a color image, each pixel has values associated and representing the magnitude or average brightness of its tristimulus color components representing its three color planes. Other image representations have more than three color components for each pixel. A different component value is associated with each different one of the image's color planes.
In what follows, whenever reference is made to color planes it is understood to include any number of color planes used by a particular image's digitizing technique to define the pixel's color characteristics. This includes the case when there is only a single plane defining a monochrome image.
A digitized image is recognizable as an image to a human viewer only when the individual pixels are displayed as dots of white or colored light on a display or as dots of black or colored inks or dyes on a hardcopy. Pixels are normally spaced so closely as to be unresolvable by the human visual system. This results in the fusion of neighboring pixels by the human visual system into a representation of the original physical image. Image fusion by the human visual system makes invisible marking, or relatively invisible marking, of images possible. This property is fully exploited by the methods described here to both impart upon a digitized image an invisible watermark to a desired invisibility classification, and to subsequently demonstrate its existence. The imparting and demonstrated detection of a robust invisible marking on digitized images, herein called invisible watermarking, are a primary aspect of the present invention.
Properties of a Robust Invisible Watermark
A proper invisible watermarking technique that imparts an invisible watermark upon a proprietary digitized image should satisfy several properties. The imparted watermark should appear to be invisible to any person having normal or corrected visual accommodation to a desired invisibility classification level. Clearly, the degree of marking is a dichotomy. A balance has to be struck between protecting the image from unauthorized uses and not having the watermark unpleasantly alter the appearance of the image. This generally means that a recognizable pattern should not appear in the marked image when the watermark is applied to a uniform color plane. This requirement discourages marking the image by varying the hue of its pixels, since the human visual system is significantly more sensitive to alterations in hue than in brightness. The requirement can be satisfied by a technique based on varying pixel brightness implemented in a proper way. A technique based on varying pixel brightness also allows the same marking technique applied to color images to be equally applicable to monochrome images.
Another property of a proper invisible watermarking technique is that it should have a detection scheme such that the probability of a false-positive detection is vanishingly small. For purposes of the present invention, the probability of detection of a watermark in an image when one does not exist should be less than one in a million. There is generally little difficulty satisfying this requirement when the technique is statistically based.
Still another property of a proper watermarking technique is that it should be possible to vary the degree of marking applied to an image. In this way, the watermark can be made as detectable as necessary by the particular application. This property is important in highly textured images where it is often necessary to increase the intensity of the mark to increase its likelihood of detection. This is in contradistinction with images that have low contrast in which it is advantageous to reduce the marking intensity to lessen undesirable visible artifacts of the watermark itself.
It is also highly desirable that when detected the demonstrated existence of the watermark should be translatable to a recognizable visual image having relatively bold features with a high contrast ratio. Features of a demonstrated visual image that are not relatively bold may otherwise be difficult to show if the watermark has been attacked in attempts to defeat its protection.
Finally, the imparted watermark should be robust in that it should be very difficult to be removed or rendered undetectable. It should survive such image manipulations that in themselves do not damage the image beyond usability. This includes, but is not limited to, JPEG “lossy” compression, image rotation, linear or nonlinear resizing, brightening, sharpening, “despeckling,” pixel editing, and the superposition of a correlated or uncorrelated noise field upon the image. Attempts to defeat or remove the watermark should be generally more laborious and costly than purchasing rights to use the image. If the image is of rare value, it is desirable that the watermark be so difficult to remove that telltale traces of it can almost always be recovered.